Polymer electrolyte fuel cells are devices in which a fuel electrode and an oxidant electrode are respectively bonded to both faces of a solid electrolyte membrane such as a membrane of perfluorosulfonic acid or the like, as the electrolyte, and wherein power is generated through an electrochemical reaction sustained by supplying hydrogen or methanol to the anode, and oxygen to the cathode. Among such fuel cells, polymer electrolyte fuel cells using methanol as fuel, called “direct methanol fuel cells (DMFCs)”, generate power in accordance with the following reactions.Anode: CH3OH+H2O→6H++CO2+6e−  [1]Cathode: 3/2O2+6H++6e−→3H2O  [2]
To support these reactions, the electrodes are made of a mixture of a solid polymer electrolyte with carbon microparticles that carry a catalytic substance.
In such direct methanol fuel cells, the methanol supplied to the anode passes through pores in the electrode and reaches the catalyst. The catalyst causes the methanol to decompose and generate electrons and hydrogen ions, in accordance with reaction [1] above. The hydrogen ions traverse the electrolyte of the anode and the solid electrolyte membrane interposed between the two electrodes, and reach the cathode, where they react with oxygen supplied to the cathode and with electrons flowing through an external circuit, to generate water in accordance with reaction [2] above. The electrons released by the methanol traverse the catalyst carrier in the anode and are led to an external circuit through which they flow into the cathode. As a result, power is extracted at the external circuit on account of the flow of electrons from the anode to the cathode.
Direct methanol fuel cells using methanol as fuel are useful as small power sources for portable electronic devices, because of their low operating temperature and the fact that they require no major auxiliary equipment, among other advantages. Efforts to develop direct methanol fuel cells as next-generation power sources for portable computers, cell phones and the like have intensified in recent years.
The methanol used as fuel, however, is a liquid, and hence prone to leaking. The flammability and toxicity of methanol itself are also grounds for concern. Coming up with ways of using methanol safely has thus become a challenge. Further demerits of using a liquid fuel include, for instance, impairment of fuel cell performance when impurities dissolved in the liquid fuel are supplied to the fuel cell, and the phenomenon of crossover, whereby methanol, as the liquid fuel component, permeates through the electrolyte membrane of the fuel cell and gets to the air electrode. In particular, crossover results not only in a drop of power generation efficiency per unit volume of fuel, but also in the formation of hazardous substances such as formaldehyde, formic acid and methyl formate in the oxidation process at the air electrode. Solving these problems has become a significant issue in the practical application of DMFCs.
The mainstream approach to increasing fuel volume density in DMFC systems developed in recent years involves using methanol at higher concentrations. However, higher fuel concentrations further exacerbate the problem of crossover. Another approach for reducing crossover relies on improving the materials of the electrolyte membranes and so forth that are used in the cells, although this approach has not been fully successful thus far. This failure has significantly hindered the commercialization of DMFCs.
To address the issue of methanol safety, among others, the present applicant has proposed therefore various “solid methanol fuels” in which methanol is made into a solid state, through formation of a molecular compound, to reduce the likelihood of fuel leaking while substantially reducing the flammability of the fuel (Patent documents 1 to 3). When the solid methanol comes into contact with water, the methanol in the solid is released into the water. The resulting methanol aqueous solution can also be used as the fuel of a direct methanol fuel cell.
Patent document 1: Japanese Patent Application Laid-open No. 2006-040629
Patent document 2: Japanese Patent Application Laid-open No. 2005-325254
Patent document 3: WO 2005/062410